API-571 Exam Dumps

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571 and API RP 941 (Nelson Curves), 1 Cr--0.5 Mo steel is no longer recommended for services susceptible to High-Temperature Hydrogen Attack (HTHA).

API RP 941 documents industry experience showing that 1 Cr--0.5 Mo steels have suffered HTHA damage below previously assumed safe operating limits, due to insufficient carbide stability. As a result, this material has been removed from the Nelson Curves as an acceptable choice for new construction in hydrogen service.

By contrast:

Mn--0.5 Mo, C--0.5 Mo, and 1.25 Cr--0.5 Mo steels retain higher resistance due to more stable carbide structures.

Referenced Documents (Study Basis):

API RP 571 -- Section on High-Temperature Hydrogen Attack

API RP 941 -- Updated Nelson Curves and Material Recommendations

Comprehensive and Detailed Explanation From Exact Extract:

Naphthenic Acid Corrosion (NAC) occurs in high-temperature refinery streams (typically 450--750 F / 230--400 C) containing organic acids. According to API RP 571, NAC is particularly aggressive toward carbon steels and low-alloy steels, and resistance improves with increasing chromium and molybdenum content, but is best with titanium.

Titanium exhibits exceptional resistance to NAC due to the formation of a stable, protective oxide film that is not attacked by naphthenic acids. API RP 571 identifies titanium as one of the most resistant materials available for severe NAC environments.

Why the other options are less suitable:

Option B (9 Cr-1 Mo steel) has limited resistance and is still susceptible at higher TAN and velocities.

Option C (317 stainless steel) offers improved resistance but can still experience attack under severe NAC conditions.

Option D (321 stainless steel) has lower molybdenum content than 317 SS, making it less resistant.

API RP 571 explicitly notes that titanium provides superior resistance compared to stainless steels and Cr-Mo alloys in NAC service.

Referenced Documents (Study Basis):

API RP 571 -- Section on Naphthenic Acid Corrosion

API Corrosion and Materials Selection Study Guide

Comprehensive and Detailed Explanation From Exact Extract:

Per API RP 571, mechanical fatigue failures are identified by the presence of beach marks or clam shell patterns on the fracture surface.

These features:

Appear as concentric rings

Represent successive crack growth increments

Radiate outward from the crack initiation site

This ''clam shell'' or ''beach mark'' appearance is one of the most recognizable signatures of fatigue failure.

Referenced Documents (Study Basis):

API RP 571 -- Section on Mechanical Fatigue

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, temper embrittlement is a metallurgical condition that affects Cr-Mo low-alloy steels, including 1-1/4 Cr-1/2 Mo, when exposed to temperatures typically in the range of 650 F to 1100 F (345 C to 595 C) over extended periods. This damage mechanism results in a significant loss of fracture toughness and ductility, particularly at lower temperatures.

API RP 939-C further explains that temper embrittlement does not significantly reduce tensile strength, but it raises the ductile-to-brittle transition temperature (DBTT). As a result, equipment that appears structurally sound may fail catastrophically under sudden loading conditions.

Once ductility is reduced, the material becomes especially vulnerable to rapid temperature changes, which induce high thermal stresses. Thermal shock is therefore a critical secondary damage mechanism. Sudden quenching, cold feed introduction, startup, shutdown, or uneven heating can cause cracking because the embrittled material can no longer accommodate strain plastically.

Option A (Ductile rupture) is incorrect because temper embrittlement promotes brittle fracture, not ductile failure.

Option B (885 F embrittlement) is incorrect because 885 F (475 C) embrittlement primarily affects carbon steels and some stainless steels, not Cr-Mo steels.

Option D (Graphitization) occurs at prolonged exposure above approximately 800 F (425 C) in carbon steels and is not the dominant concern for 1-1/4 Cr-1/2 Mo steel in this context.

API RP 571 explicitly emphasizes that embrittled Cr-Mo steels are highly susceptible to cracking during thermal transients, making thermal shock the most likely and dangerous subsequent damage mechanism.

Referenced Documents (Study Basis):

API RP 571 -- Section on Temper Embrittlement of Low-Alloy Steels

API RP 939-C -- Metallurgical Effects and Service Risks of Temper Embrittlement

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, hydrogen embrittlement (HE) of carbon steels is a low-temperature damage mechanism. It occurs when atomic hydrogen enters the steel lattice, reducing ductility and toughness and potentially causing cracking or delayed failure.

API RP 571 clearly differentiates HE from high-temperature hydrogen damage mechanisms (such as HTHA). Hydrogen embrittlement is most severe at ambient to moderately elevated temperatures, typically below about 200 F (93 C), where hydrogen mobility and trapping promote embrittlement.

At higher temperatures, hydrogen tends to diffuse out of the steel and embrittlement effects diminish.

Referenced Documents (Study Basis):

API RP 571 -- Section on Hydrogen Embrittlement